Railway signaling



Aug. 2s, 1934.

L.` ESPENSCHIED RAILWAYSIGNALING Filed Feb. 19, 1924 @ik ESS INVE-NTOR Z3 ewezea ATTONEYS Patented Aug. 28, 1934 UNITED STAT-ES.

PATENT-2 OFFICE 40 Claims.

This invention `relates to railway signal and control systems.

In a broad sense, the present invention emvploys the same fundamental principle as that 5 disclosed in my co--pending earlier vapplication for patent, Serial No. 339,658, filed November 21,

1919, now Patent No. 1,517,549, dated December 2, 1924. In the system there described, the signal impulses are transmitted ahead ofthe train (or other point at which the signal is located) and subjected to control in accordance with trafc conditions, the impulses being returned to the train under certain conditions and not returned under other conditions, the signal and control l5 apparatus on the vehicle being governed' accordingly.

The present application diiers from the method of operation and arrangement of parts disclosed in the said previous application in sevilo eral respects, but particularly in that the present invention lproposes to distinguish between the outgoing and returning currents by differences in frequency, whereas the method specifically vdescribed inv my original application made this differentiation by means of electrical balance.

In general, the present invention proposes to transmit from the point of signal location a current of a given frequency, herein termed the original frequency, and to transmit this frequency to the point of signal control. Dependent upon trafiic conditions and in accordance with the signal to be conveyed, current of a frequency differ- A-erit from said original frequency, is sent from the point of signal control to thevpointof signal location. At this point the current of different frequency is separated from the current of original frequency and caused to govern the signal or control apparatus.

ln the particular embodiments of the inven- -10 tion herein described, the change of-frequency at the point of signal control is effected by modulation of the current of original frequency by current of another frequency to provide a current of a third frequency which current is then returned to the signal location and caused to operate the signal.

The current thus returned to the signal location is differentiated from the current sent out from said location, namely, the current of original frequency, by frequency selective circuits.

In case the outgoing and returning frequencies cannot readily be `separated from each other by means of ordinary tuned circuits by reason of the 'fact that the frequencies differ fromv each otherbut slightly, the invention provides means for converting the frequencies in such manner that the converted frequencies may readily be distinguished from each other.

, The method is specifically applicable to cab signaling systems for the reason that the original frequency may be generated on the locomotive and transmitted along the trackway to the point of signal control, e. g., the entrance of the next block ahead, at which point the modulation is effected in accordance with traffic conditions in 65l the said block. The current resulting from this modulation is then sent back from the point of signal control to the locomotive, at which it is received, segregated from the original frequency, and caused to operate the cab signal or control apparatus.

The invention further contemplates to employ signal or control apparatus, the inherent tendency of which is to assume a danger indication or control so that a safety indication can be givenonly if the proper operating current is continually being received from the wayside. This current, however, is interrupted or not generated at all in case of failure of any part of the signaling system, so that the signal apparatus is auto- 8.0 matically set at the danger indication and the control apparatus is automatically caused to stop or otherwise control the train.

The method of the present invention may be readily applied to existing automatic signaling systems by merely providing the necessary sending and receiving apparatus on the cab of the 'locomotive and the return apparatus at the points of signal control. The apparatus ordinarily employed for the automatic control of wayside signals is retained without substantial change. The invention, therefore, does not necessitate the discardment of wayside signaling apparatus employed in the usual present-day automatic signaling systems, but necessitates merely thev installation of additional equipment which is comparatively low in cost.

A good understanding of the invention may now be had from the following description thereof, having reference to certain preferred embodiments shown in th accompanying drawing, in which Figure 1 is a diagrammatic view illustrating one form of embodiment of my invention;

Figure 2 is a view' showing a modification of 105 Figure 1, and

Figure 3 is a View showing a modification of the cab circuits vshown in Figure 2. v

Similar characters of reference designate similar parts in each of the several views. no

In Figure 1, reference character l1 designates the rails of a railway track, which rails are divided into sections A, B, C, etc., by means of insulated joints.

'I'he railway vehicle indicated at 3 is provided with a source of currentl S connected to coils of inductorium 4 placed in inductive relation with the track rails. Loosely coupled tuned circuits 5 and 6 are interposed between source S and coils of inductorium 4 so that current of only a single frequency is transmitted into coils of inductorium 4, and from these coils into the track rails 1. In the illustrated embodiment coils of inductorium 4 are shown as consisting of a pair of coils a and b in series with each other, each coil being wound on an open magnetic core c, the open side of which is placed into as close relation to the rails of the track as good railway practice will permit. Said coils 4 are thus associated inductively with the rails of the track and serve to impress single frequency current from source S on the rails of the track.

In Figure 1 the vehicle 3 is shown as occupying section B. The insulated joints 2 confine theA current impressed on the track to section B. At the exit endof the block the current enters a modulator M which may be of any type well known in the art, as, for example, of the magnetic or vacuum tube type. The input end of the modulator is connected to the track rails thru conductors 7 and transformer 8, condensers 9 being inserted yin this connection for purposes of sharp tuning. The source of current for modulating the current impressed on the modulator is indicated by reference character E. The frequency of this modulating current bears a predetermined relation with reference to that of source S.

Although the invention is not limited to the employment of any particular frequencies, it may be assumed for the purposes of the present description, that source S has a frequency of 8,000 cycles, and source E a frequency of 3,000 cycles. By reason of the modulation, the output of the modulator will comprise currents of several frequencies, prominent among which is that of the difference frequency, namely 5,000 cycles.

This current will be transmitted through transformer 10. tuning condensersy 11, and through connection '7 into the track rails of section B. By these rails the current is conducted to the railway vehicle at which it is picked up by coils 12 and transmitted by sharply tuned circuits 13 and 14, an amplifier 15 and a rectifier 16 to a control relay 1'7. The amplifier and rectifier are indicated diagrammatically, but it will be understood that they may be of any suitable form, preferably of the vacuum tube type. Control relay 17 operates suitable signaling and control apparatus on the railway vehicle, which apparatus may be of any well known type, preferably of a character such that it has an inherent tendency to assume its danger position in case of failure of operating current.

It will thus be seen that in the present embodiment of the invention the current for the operation of the signal is generated on the vehicle, transmitted through the track circuit to the distant end of the block, there modulated by current of another frequency to produce current of a third frequency, which current is returned to the vehicle to effect the signal and control operation. The transmitting and receiving circuits in the cab h circuit are sharply tuned to their respective frequencies, so that the currents are segregated and a false operation of the signal -is effectively prevented.

It will be further observed that in 'case of a broken rail in block section B, or in case another railway vehicle is present in this section, the transmission circuit for the signaling current is interrupted .or short-circuited, as the case may be, so that current from source S cannot reach the modulator and no modulated current of the said third or difference frequency can be set up. Consequently, in case of danger conditions in block section B, no current of the proper frequency. can be received in the signal circuit of the vehicle, so that the signal will be set at its danger indication and the control apparatus will be rendered effective to slow down and stop or otherwise control the vehicle, according to the -particular kind of control apparatus which may train in the section next in advance to that occupied by the train, namely section C. In the illustrated form of embodiment, this control is effected by the relay 19, which relay is bridged across the track rails of section C and constitutes part of an ordinary track circuit comprising a source of current bridged across the rails at the distant end of the block similar to the source X shown for block section B. Relay 19, therefore, is energized or de-energized according to whether this section is clear or occupied. In the energized position of the relay, contact 20 of relay 19 closes the input circuit to the modulator M, so that current from source S may enter the modulator. In the deenergized position of relay 19, however, contact 20 is open, so that the current from source S is unable to enter the modulator. No current of the said third frequency, therefore, will return to the vehicle and a danger indication of the signal will be effected. 'Ihe signal is thus controlled by traflic conditions, both in the block occupied by the train in the block next ahead.

It will readily be seen that by a simple extension of the principle above described, any desired condition may be caused to govern the signal, and that the control may be manual or automatic, the general term traffic conditions being employed in the present specification to denote any conditions of control, no matter how effected.

It is to be understood that the frequencies stated are merely illustrative and are not to be regarded in a limiting sense. For some applications the best frequency may be measured in kilocycles, for others it may be as low as a few hundred cycles. y

The use of the track as part of the wayside circuit is not a necessary feature of the invention, as any other suitable conductor may be employed.

The fact that the present system is such that the track rails may be employed in the wayside circuit is, however, a feature of substantial value.

In the embodiment illustrated in Figure 2, the railway track, as before, is divided into sections of which four are shown, namely, A', B', C', and D'. The railway vehicle is provided with a source of alternating current S which source is connected through a tuning condenser to the coil 4'. At the exit end of the block this current enters a modulator M which, in the specific form of embodiment, is shown as being of the magnetic type. It will be understood however, that any` other suitable type of modulator may be employed. In the illustration the modulator is shown as provided with windings 21 and22, the former of which is connected through leads 25 to the track rails, and the latter of which is connected through v leads 26 to the source of current supply E', shown in the drawing as the transmission line 27, sup-v plied with alternating current by a generator F. This line serves also to convey polarizing current to the modulators from direct current generator G, inductances 'Ii'tbeing formed to minimize the flow of alternating current through generator G and a transformer '10' including acondenserbeing furnished to exclude the direct current from generator F. The modulated -current from M', i. e., the current of the third or difference frequency, 4is returned through leads 25 to the track rails and from there enters the coil 4' and source S. At this point the current of thedifference frequency is segregated from the current of original frequency and caused to effect the desired signal or control operation. This segregation may be effected by tuned circuits as in Figure 1, provided the percentage difference in the frequencies of f 'the currents is sufficiently great. Where this clifference is insuflicient, however, the invention provides a means for sovconverting the frequencies that they may be separated by properlytuned circuits. For purposes of illustration, it will be assumed in the present example, that the frequency of source S is 5000 cycles and the frequency of source E' is 60 cycles, so that the sum and difference frequencies returned to the vehicle from modulator M are 5060 and 4940 cycles. The percentage difference, therefore, between the outgoing and returning frequencies is very slight and ordinary tuned circuits cannot well be relied upon to effect their segregation. 'I'he present invention,` however, proposes to so convert these frequencies that they may be readily segregated from each-other. The apparatus for effecting this conversion of the frequencies will now be described.

In the illustrated form of construction source S' comprises an alternator 28, having a field winding 29 and a direct current exciter 30, the current from the exciter being impressed on the field through leads 31. The current received by coil 4' of the vehicle enters the circuit of the armature of alternator 28, and passes by induction into the field winding 29.

However, the relative motion between armature and field effects what is essentially a modulation or what might be termed a demodulation or detecti'on of the magnetic fields set up in the armature by the received currents. Thus the alternating armature magnetic eld generated by the received currents is modulated at a 5000 cycle rate, which is the frequency equivalent of the rotational speed of the machine.

As is now well known, the effect of modulating received currents of one set of frequencies in accordance with another frequency is to set up two new sets of frequencies, one representing the original set of frequencies plus and the other the original set minus the frequency of modulation. Thus, for each of the two frequencies which are returned from modulator M', two new sets of frequencies will be generated in the field winding. In each case the components representing the sum termsl are not useful and can be neglected. The components representing the difference terms come out to be 5000-4940=+60 cycles, and 5000-50.60=-60 cycles. The phase relations are such that the +60 and the -60 cycle components are additive in the eld winding, so that a 60 cycle current is set up. The components of the return transmission which are used to convey the signal inciicationare, therefore, transferred over to the field windings as a 60 cycle current with the originally generated current of 5000 cycles completely eliminated and absolutely unable to get into the receiving channel.

, The current thus derived is employed to govern signal and control apparatus not shown in thedrawing. an alternating current relay 33 being employed for eecting this control.'l Separation of the `direct current from the exciter and the alternating current from the field winding is effected by means of a condenser 34 in series with the relay and an anti-resonant circuit 35in series with the exciter armature. The condenser 34 also tunes the circuit of the relay-to obtain the maximum flow of current. A suitable amplifier, not shown in the drawing, may be provided in the circuit between the field winding 29 and relay 33, in order to increase the power available-for the operation of the relay.

It will be observed in the embodiment of Figure 2 that the modulator M' is controlled by track circuit apparatus illustrated by relay T' bridged across the track rails and supplied with current by the track circuit energized from transmission line E' through the transformer 70, the secondary side of which is connected to the track rails by the leads 25. Choke coils 7l and condensers '72 are provided in this connection to prevent the high frequency current entering and leaving modulator M from entering transmission line E', and to prevent the low frequency current of transmission line E from entering the modulator M', withoutbeing subject to the control of the traffic conditions ahead.

It will thus be observed that in the present form of embodiment of the invention the ordinary low frequency source of signaling current is employed for two purposes first to control the track relays T', and second to modulate the original current transmitted from the vehicle in order to provide the desired third frequency of return transmission. The source of low frequency is illustrated in Fig. 2 as that of an ordinary electric power supply circuit. It will be realized that this is merely a convenient low frequency source and the exact nature of thesource is-not germane to the invention. What is essential to the invention is that there be provided in the wayside a source of frequency or periodic variation of some kind in a form capable of being impressed upon the higher frequency current supplied to the track,`

and, in turn, capable of being detected as a low frequency current on the train. The requirement as to the frequency of this variation is that it be low compared to the frequency of the current upon which it is impressed.

The advantage of having the superimposed frequency low compared with the carrier frequency is two-fold:

(l) The band of frequencies required to be picked up by the locomotive system is then so narrow that all the frequency components, carrier and so-called side frequencies, can efficiently pass through one and the same train-'carried tuned inductor system, instead of requiring two such inductor systems, as in the arrangement in Fig. l.

(2) The'current which is demodulated or detected" on the locomotive for operation of the control relays can be the more sharply selected from other currents, the lower it is in frequency.

The reason why greater selectivity can be obtained at a low frequency goes back to a fundamental characteristic of tuned circuits which can be recalled by reference to such circuits as they are employed in the locomotive portion of Fig. 2. The inductor circuit 4', 50, 28l is tuned to select the higher frequencies as they'are picked up from the track. In practice the selectivity of such a circuit may prove to be 2%, meaning that it has a prescribed degree of suppression of undesired currents at frequencies removed from the resonant frequency 42% or more. With, a.y resonant frequency of 5000 cycles, this means that the prescribed degree of suppression is attained outside of a band of 100 cycles either side of the resonant frequency. This is a relatively wide band for railway signal purposes and for obtaining the necessary protection against false operation. Now, compare with this the performance of a similar tuned circuit operating at low frequency. Closed circuit 29, 34. 33 is such a circuit. It may very well have a somewhat better selectivity, say 1%. This percentage applies to 60 cycles and the band outside of which the desired exclusion is realized is then 60 1%, or 0.6 cycle either side of the resonant frequency. Thus, the sharpness of the selectivity and the protection aiforded against undesired frequency components and, in turn, false operation, is greatly improved by the application of frequencyl selective elements at the low signaling frequency, as distinguished from the higher transmission frequency. In the case of Fig. 3 the condenser shunted around the output transformer is a selective shunt, operating in combination withthe transformer as a low-pass filter to protect the low-frequency output circuit against higher frequency components. In Fig. 2 and Fig. 3 the locomotive receiving circuit has the protection of both high-frequency and low frequency selectivity, an important combination. The high-frequency selectivity gives an overall protection against foreign influence, particularly as against two frequencies coming through and being demodulated down to very low frequencies, and thus getting past the low-frequency selectivity. The low-frequency selectivity protects against the higher-frequency components which may pass through pr be generated in the higher frequency stage.

The importance to the inductive type of traincontrol system of the combination of a relatively high transmission frequency for obtaining the necessary inductive action, of stepping down to a relatively low frequency and nally of utilizing the low frequency for obtaining a high degree of selection will, therefore, be appreciated.

Figure 3 illustrates a modification of the apparatus carriedby the vehicle. Current of the desired frequency, in the present instance 5000 cycles, is generated by the vacuum tube oscillator consisting of an electron tube 81 provided with an input circuit consisting of a portion of the turns of the track coil 4' connected between the filament and the grid, and .an output circuit comprising the remaining turns of the track coil 4 and B"battery 82 connected between the filament and the plate. -A condenser 83 is connected in shunt to coil 4 and bears such relation to the inductance of coil 4' that the oscillations are of the desired frequency, namely, 5000 cycles per second. 'I'he grid of the oscillator is made suiciently negative by means of a grid condenser 84 to cause the tube to function also as a detector. The current thus 4generated is transmitted into the wayside circuit and travels toward the distant end of the block, as in the case of Figure 2. The returning frequencies of 5060 and 490 cycles enter the coil 4 and pass through thecircuits of the oscillator, and by virtue of modulation therein with the 5000 cycle current being generated, are converted into a 60 cycle current, which current is separated from the direct curren't component of the plate current by a transformer and is caused to energize an alternating current relay 85 84a. This relay is connected to asuitable signal and control apparatus not shown in the drawing. An amplifier of any suitable type, indicated in the drawing by reference character 85, may be provided for reinforcing the received current.

It will be noted that in both the embodiments illustrated in Figures 2 and 3, the generator of the outgoing or original" frequency current acts simultaneously as a modulator or detector of the returning current so as to insure the segregation of the one current from the other. By virtue of this twofold function of the generator, any uctuations.` in the frequency of the outgoing current are automatically compensated for by virtue of the fact that the same fluctuations are simultaneously taking place in the modulation of the returning current, so that thejfrequency of the relay operating current remains unaffected.

Although I have herein shown and described only certain specic forms of embodiment of the invention, it will be understood that the same is not limited thereto, but that various changes and modifications may be made therein Within the scope of the accompanying claims, without departing from the spirit and scope of the invention.

What I claim is:

1. The method of trafilc signaling herein described, which consists in sending out from a signal location a current of one frequency, causing the return to the signal location of current of a different frequency for controlling the signal and controlling said return in accordance with traflic conditions.

2. The method according to claim 1, in which ifm the current of the different frequency is set up by tl're modulation of the current of the rst-mentioned frequency with current of another frequency.

3. The lmethod according to claim 1, in which 9' the current of different frequency is set up by the modulation of the current of the first-mentioned frequency with current of another frequency and the actuation of the signal is effected by current of said other frequency produced by modulating U" said current of different frequency with current of the first-mentioned frequency.

4. In a railway signaling system, a wayside circuit, means on a vehicle for transmitting energy of certain frequency into said circuit, means in 1135 the energy of said'different .M

the frequency of the returning transmission with respect to ,theoutgoing transmission, and means at the'signal location for segregating saidreturning transmission from the outgoing transmission.

6. In a railway signal system, a wayside circuit, means on a vehicle for impressing on said circuit an outgoing 818ml transmission of certain frequency, means in the wayside circuit for setting3up and returning to the vehicle a signal tr on the vehicle selectively responsive to said returning signal transmission.

'-7. The system claimed in the preceding claim,

'in which the trackway is divided into a plurality and in which means is provided for governing the return transmission of one section in accordance with traffic conditions inanother section.

9. The method as in preceding claim 6, in which the means for setting up the return transmission comprises a source of current differing in fre@ quency from said certain frequency and means `for modulating said certain frequency by said current of different frequency.

10. A system as 'setforth in claim ii, in which the trackway is dividedvinto a plurality of bloot; sections and in which means for effecting said return transmission is provided for each section and in which meansresponsive to traic condi-1l tions in one section is provided for governing said means for effecting the return transmission in another section.

l1. In a railway signaling system, a wayside circuit, means at a signal location for impressing on said circuit an outgoing transmission of waves ofI certain frequency, means in said wayside circuit, at the end thereof remote from said signal location, for governing said transmission to set up a return transmission of different frequency, and means 'at the signal location responsive to said different frequency.

12. A railway signaling system, means for dividing the trackway into a plurality pf blocks, a wayside circuit for each block, means on a l vehicle for impressing current of a certain frequency on saidA wayside circuit, means adjacent the exit end of each block for modulating said current by current of different frequency and impressing on said wayside circuit a current of a third frequency, and means on said vehicle responsive to said third frequency.

13. In a railway signaling system, means for dividing the trackway into blocks, a Awayside circuit for each block, the wayside circuit of each block being responsive to' traflic conditions in `another block, means on the railway vehicle for A' `current to the point of signal location, modulating the last-mentioned current with-the original frequency to derive current of said second frequency and governing in accordance with the' signal to be conveyed the modulation of said original frequency with .said signal frequency.

15. In a railway signal system, a source of current of original frequency at a point of signal location, means at a point of signal control for ionk of different frequency and means' modulating said current of original frequency with a current of signal frequency, means for governing said modulation in accordance with the signal to be conveyed, means for transmitting the so modulated current to the point of signal location,.-and signaling means responsive to said current.

16. In a signal system, a source `oi current of original frequency at a point of signal loca,- tion, a source of current of signal'frequency, means at a point of signal control for subjecting said current of original frequency to modulation with current of said signal frequency, means for governing said modulation in accordance with the signal to be conveyed, means for transmitting the so modulated current to the point of signal iocation,y and means for modulating the lastmentioned current with the original frequency te derive said current of signal frequency, and signal meansresponsive ,tosaid signaling frequency.

it. A railway signal system comprising a source of current of` predetermined frequency, means for impressing said current on a trackway, means subject to traffic conditions for modifying saidA current to derive current of changed frequency, means for modulating said current of changed frequency with the frequency of the said source of current, and means responsive to the frequency derived from said modulation.

18. The method of railway signaling which consists in sending out current of pre-determined frequency to a point of signal control, modulating the current; of said frequency with va. current of signaling frequency, and transmitting the modulated current to a signal responsive device.

is. The method of signalingwhich consists in transmitting current of pre-determined frequency to a point of signal control,` modulating said current with current of a second frequency, transmitting the so modulated current to a point of signal location, and subjecting the last-mentioned current to modulation by the pre-determined frequency to derive therefrom current of said second frequency.

Y 20. In a railway signalsystem, a generator of alternating current, means for receiving current from said generatorl and for returning thereto current of different frequency, and means for deriving from said generator current depending upon the difference in said frequencies.

2l. In a railway signal system, a signal and control device having an inherent tendency to assume a danger position, means at a signal location for impressing current of pre-determined frequency on the trackway, means subject to traflic control for returning current of'different frequency to the signal location, said current operating to restrain said signal and control device from assuming its danger position, so that failure .of any part of said system will cause said signal and control device to revert to said danger position. e

22. In a train control system, a source of alternating current of definite frequency, means 14()- in a wayside system for converting said current into current of a different frequency, means for controlling said current in accordance with trafc conditions, and means for inductively transmitting the current of converted frequency into t0 Send to said vehicle signal energy of a diifer- 150 ent frequency. a signal on said vehicle normally set to danger but responsive to said signal energy -to move to a clear position, the operativeness of said wayside circuit to send signal energy being dependent upon the block ahead being clear and also upon the current from said source on the vehicle reaching the wayside circuit, and means continuously coupling the vehicle for the transmission ofcurrent from said source to the wayside circuit and for the transmission of signal energy from the wayside circuit onto the vehicle.

24. The method of giving continuously on a train an indication of` traffic conditions ahead which consists in continuously transmitting energy into a wayside conductor, under clear trafiic conditions ahead effectingby such energy a superimposed continuous return transmission over the same wayside conductor to the train of such .character as to be distinguishable from the outwardly directed transmission, continuously receiving said return transmission on the train to the exclusion of the outgoing transmission and actuating thereby a signal-receiving device to give a clear" indication, and finally upon the occurrence of other than "clea conditions ahead, suppressing the return transmission thereby releasing said signal-receiving device and giving an other than clear indication.

25. The system for giving continuously on a train an indication of traffic conditions ahead which comprises a source of electric energy and a receiving device therefor both located on the train, indicating means on the train responsive to the receiving device, a circuit intervening between the source and the receiving device, said circuit providing over one and the same pair of wayside inductors two transmission channels, one extending from the source on the train to the wayside and the other extending from the wayside to the receiving device on the train, and means in the wayside for insuring the electrical separation of the two channels, means for connecting the two channels together in the wayside to establish continuous transmission of energy from the source through the wayside and back to the receiving device to give a clear indication, together with means in the wayside for suppressing the return transmission for an other than "clear" condition of the track ahead.

26. A train-control receiving device responsive to the coaction of two currents of different frequencies, means on the train for supplying current of one of said frequencies, and means in the wayside for generating current of the other of said frequencies in predetermined frequency relation to current o f said first frequency.

27. In a train-control system, .a train-carried source of E. M. F. and inductive means for impressing same upon a wayside circuit, means in the wayside for setting up a` periodic reactive eifect to said E. M. F., and actuating by said effect a train-carried receiver, together with means for controlling said effect in accordance with trafilc conditions ahead.

28. The method of signaling which consists in inductively impressing from a train an` excitation E. M. F. upon a wayside circuit, effecting in the wayside a periodic. modification of said periodicv modification back to the train and controlling said modified E. M. F. in accordance with traffic conditions ahead.

29. In combination, a block section of railway track, means for supplying said section withY alternating current, means in the wayside for imand transmitting the effect of said pressing a periodic modification upon said current at a frequency low compared withY said first frequency, together with means on a train operating synchronously with respect to said first frequency to receive the modified current from the railway track and to govern the train in accordance with'said periodic modification of low frequency.

30. In a system for the transfer of signal indications including a point at which signal indications are imparted to the system and a second point at which the signal indications are delivered from the system, the combination which comprises (1) an electric-circuit extending between said points, (2) a source of alternating current included in said circuit at or near the point at which the signal indications are delivered from the system, (3) means at said first mentioned point which impart signal indications to the system' by effecting a periodic modification of the alternating current in the circuit, and (4) means at said second mentioned point for converting said periodic modifications into signal indications.

31. A railway signaling system including means for dividing a trackway into a plurality of block sections, means for impressing alternating current of predetermined frequency upon each section, means adjacent the exit end of each section for modifying said alternating current into periodically recurring groups of alternating-current impulses corresponding to one condition of signaling, receiving means on a train synchronous to the unmodified alternating current, together with means for detecting, separating out and operating a receiving relay by the frequency, of the periodically recurring groups.

32. The method of transmitting signals onto a moving train which consists in providing a series of block circuits normally inert, impressing successively upon each block circuit from the train an alternating current substantially higher in frequency than cycles per second; impressing upon such current successively in each block section a variation of relatively low frequency, and operating on the train, by said low frequency, a signal-receiving device.

33. In a signaling system, a generator of alternating current of one frequency, a transmission .circuit including means for impressing upon said current a variation of a second frequency, and transmitting said variation back to the generator, means in the transmission circuit for controlling said second frequency in accordance with signal or control indications, together with means, including said generator, for deriving back said lower frequency and actuating thereby a signal indicating or control device.

34. The method of effecting the continuous control of a train in accordance with traffic conditions ahead which consists in continuously inducing energy into a pair of wayside inductors extending the length of a block, for the clear 35. The method of effecting the continuous control of a train which consists in (l) causing the entry of a train intoa block section to energize the track rails with an alternating carrier current higher in frequency than cycles per second, (2) at the exit end of said track section modifying said current at a frequency lower than thc carrier, and using the modified current to convey the train controlI indications, (3) impressing the modified current continuously upon the track rails and inductively receiving it upon the train and, finally, (4) converting the modified current to the low frequency impulses originally impressed upon it, sharply selecting said low frequency impulses and operating thereby a train control device.

36. Train carried means for receiving inductively transmitted control current which comprises a vacuum-tube detector provided with an input circuit tuned to a predetermined frequency, and an output circuit designed to pass a substantially lower frequency, a transformer anda condenser included in said output circuit and shunting the primary winding thereof, whereby a low frequency detected current is obtained, and means for repeating said low frequency current at enhanced energy and operating thereby a signal control device, l

37. The method of receiving upon a train low I frequency control impulses which are transmitted inductively at a predetermined higher frequency, which consists in (1) providing upon the train a source of current differing from the received higher frequency current by the frequency of the low frequency control impulses, (2) demodulating the received higher frequency current by means of the train-carrier source, (3) selecting the said low frequency impulses to the exclusion of lcurrents of other frequency and operating thereby a signal receiving device.

A38. In a train control system, the method of transferring signals from the wayside to the lo- ;r comotive and of selectively receiving such signals,

which consists in transmitting two frequency components, differing by a frequency small compared with the transmission frequencies, inductively receiving said components upon the locomotive, setting up an interaction between said components to produce the difference-frequency" current, selecting said low frequency current by means of a low frequency tuned circuit, and, finally, actuating thereby a relay device.

39. In a railway signaling system the combination with a train-carried equipment comprising a source of alternating-current and a signal receiving device, of a pair of wayside inductors, wayside traffic controlled means, a circuit extending between said inductors and said traiiic controlled means, and a second Wayside circuit so superimposed upon the first circuit as to enable both inductors to receive alternating current from and simultaneously to selectively transmit it to the train-carried equipment.

40. A system for transmitting signals across a single inductive coupling established between a moving vehicle and the wayside which includes: on the train, a source of alternating current of one frequency, a signal receiving device responsive to a current of a different frequency, and a tuned inductor for simultaneously transmitting ,current to and receiving current from the wayside; in the wayside, a circuit inductively coupled with the train-carried inductor and including means for effecting, in accordance with control from a block ahead, a return transmission to the train over the same wayside circuit at a frequency differing so little from the frequency of the current sent from the train as to cause both transmissions to come within the tuning band of the train-carried inductor; together with means on the train whereby the signal receiving device is made responsive only to the return transmission.

LLOYD ESPENSCHIED. 

